What is Gas Metal Arc Welding (MIG/ MAG Welding)
Gas Metal Arc Welding (GMAW) is also known as Metal Inert Gas Welding or metal active gas welding (MIG/ MAG welding). It is a semi-automatic welding process. In this process, continuous solid wire electrode is used in the form of wire as a welding consumable. Externally supplied shielding gas such as Argon, Helium, Carbon dioxide or a mixture of these gases are used for gas shielding. These gases shield the solidifying weld pool from atmospheric contamination to avoid weld oxidation and porosity.
GMAW (MIG/MAG) Process Principle
MIG/MAG is welding one of the important welding process in fusion welding processes. This process is very versatile and make it suitable to weld both thin sheets and thick sections. Welding can be performed in all positions with high productivity. As shown in figure 1, an arc is generated between the end of the solid wire protruding out of the nozzle and the work piece. The heat generated melts both the work piece and filler wire to form a weld pool.
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GMAW (MIG/MAG) Equipment setup
The welding process works on constant voltage principle. The work piece is connected to the negative terminal and welding torch to the positive terminal. The on/off switch is located on the welding torch. In on mode, the wire is continuous fed via the wire feeder. Welder moves the welding torch along the welding joint line or axis. Two types of filler wire are available in the market for GMAW process:
- Solid wire (AWS A 5.18 for carbon steel electrodes)
- Metal cored wire (AWS A 5.36 for carbon steel electrodes and low alloy steel)
Different size of filler wires are used ranging from 0.8 mm to 1.6 mm in the industries according to the requirements. A higher diameter offers more deposition although less suitable to weld thin sheet metals.
The equipment needed for GMAW set up are as shown in figure 2 are:
- Power Source
- Filler metal feeding Mechanism
- Welding Gun
- Shielding Gas
- Welding cables / leads and gas hoses
Figure 3 shows the construction of welding torch along with main components. Parts such as contact tip, nozzle, gas diffuser, liner are replaceable thus offer a cost saving in case of damage to any of these components instead of full torch replacements.
Gas metal arc welding torch is having two variants:
- Air cooled welding torch
- Water cooled welding torch.
An air-cooled welding torch is used for normal duty welding activities. Welding requiring a high duty cycle and high welding current are normally uses water-cooled torched. For example in robotic welding MIG welding and thick plate joints welding. The air-cooled torches due to their simple construction are cheaper than water-cooled torches. They are also easy to repair compared to the latter ones.
Metal transfer mode in MIG/MAG Welding
Metal transfer mode mean the form of metal transfers from filler wire to the weld pool is defined by the welding parameters, shielding gases type and operating feature of the welding power source. The four-principle mode of metal transfer in gas metal arc welding arc:
- Short circuit transfer mode : Short-circuit transfer mode happen when the welding current and voltage are low. Hence, this mode offers low heat input in the weld. This mode is beneficial for sheet metal welding, root run welding in butt joints, out of position welding and for low heat input weld depositions. The normal current for short circuit mode is 90-180 ampere and voltage within the range of 16V to 22 volts.
- Spray transfer mode : This mode requires a high current and voltage to melt the filler wire faster to avoid the contact with the base metal during welding. This mode offers advantages of high weld deposition, improved weld profile but this mode is not good to perform out of position welding. The metal transfer happens in the fine metal droplets deposited to the weld pool. For spray transfer mode to occurs two conditions shall be met:
- Minimum 80% argon in the shielding gas
- Minimum 220 ampere and 20 voltage
- Pulsed transfer mode : This mode uses a modified wave form of the output current. This offer a controlled metal deposition enabling controlled heat input, enhanced weld profile with very low spatters. Pulsed mode is beneficial for welding thin sheets, exotic materials and out of position welding.
- Globular transfer mode : Globular transfer mode takes place with high current and 100% CO2 gas in the shielding. The metal transfer take place in the form of irregular metal transfer. This mode is beneficial for thick plate joints. The disadvantages are high spatters and high heat input. Because of this reason, it is not recommended for sheet metal welding.
Shielded gases for MIG-MAG Welding
Shielding gases protect the weld pool from the adverse influences of the atmospheric contamination. Shielding gas for GMAW is classified into two groups:
- Inert gases such as argon & helium, GMAW using inert gas is called Metal inert gas (MIG) welding
- Active gases such as CO2, Ar + CO2 mixture, GMAW using inert gas is called Metal active gas (MAG) welding
CO2, argon + CO2 and CO2 + argon + oxygen mixtures are used for carbon steel and low alloy steel welding. Usually inert gases such as argon and argon + helium mixtures are used for the welding of non-ferrous materials.
Advantages of GMAW (MIG Welding)
- GMAW process can be carried out using different metal transfer modes for various welding conditions.
- GMAW is a versatile process and used to weld most engineering materials such as carbon steel, low alloy steel, stainless steel, aluminum, copper, and nickel alloy, etc.
- The process offers high productivity usually 8-10kg deposition is made per shift. Very high productivity is possible using the tandem MIG option.
- The process can be used to weld in all positions.
- Due to the continuous electrode feeding mechanism, we can achieve higher welding speeds and higher filler metal deposition rates than shielded metal arc welding (SMAW)
- Due to the continuous electrode (wire feed) feed, longer welds can be deposited without intermediate stops and starts. This also minimizes the start-stop points cleaning or grinding.
- The process being flux less offers minimal post-weld cleaning.
- Since the GMAW is a semi-automatic welding process, it is easy to learn for welders.
- This process can be easily automated for robotic welding and for special purpose machines (SPM).
Limitations of GMAW (MIG Welding)
- The GMAW equipment is more complex, costlier, and less portable compare to shielded metal arc welding (SMAW).
- The weld metal solidifies faster as compare to SMAW and SAW and can result in degraded weld metal properties.
- Shielding of the molten weld pool is quite difficult during windy and drafty environments. High chances of porosity in the weld metal during windy environments. This restricts the process for indoor welding only.
- The equipment setup is not suitable for outdoor work.
Applications of MIG-MAG Welding
GMAW welding is widely used in the various industries and account for a major weld deposition made worldwide. It is being used in automobile, railway, structural, heavy engineering and oil & gas areas.